I had an ECG a couple years ago after having had an ablation (~4 years ago), and mitral valve repair (coming up on 2 years ago). The cardiologist said all is good, but as with your experience, the scan said I was in heart failure. The next time I saw the cardiologist, he said that sometimes the machine is not very good a reading the ECG.
Since you are still having symptoms, I would get in when you can and follow up as recommended.
All the best.